The subject matter disclosed herein relates to the art of turbomachines and, more particularly, to a heat recovery steam generator for a turbomachine.
Conventional combined cycle power plants employ a gas turbine system operatively coupled to a steam turbine system. The gas turbine system includes a compressor coupled to a gas turbine. The steam turbine system includes a high pressure (HP) turbine portion operatively coupled to an intermediate pressure (IP) turbine portion that, in turn, is coupled to low pressure (LP) turbine. Generally, the HP, IP and LP turbines are employed to drive a generator. In a typical combined cycle power plant, exhaust gas from the gas turbine is passed to a heat recovery steam generator (HRSG). The HRSG can have one, or multiple pressures, For a three pressure system the HRSG includes three different pressure heaters corresponding to three steam turbine pressures, e.g. HP, IP, and LP for a high performance combined cycle power plant. The HRSG also receives low energy steam from the HP steam turbine exhaust passing from the HP steam turbine. The low energy steam is used to reheat steam in the different pressure heaters for enhanced efficiency. The reheated steam is then passed back to power a lower pressure stage of the steam turbine.
Current combined cycle power plants are slow to move from rest to operational speeds. That is, at present, the time required to bring the gas turbine into operation, ramp the steam turbine up to speed and operate the HRSG is substantial. Shortening the start up time, i.e., fast starts, leads to increasing stress and cycling effects for the HRSG that leads to critical problems. In addition, multiple starts/stops resulting from periodic changes in demand also creates detrimental stresses within the HRSG. One such stress is caused by a quenching effect that occurs during HRSG purge.
For combined cycle power plants, a required HRSG purge can be done either immediately prior to plant start up or right after shutdown. The purge leads to a large amount of condensate that causes a quench effect in a superheater header portion of the HRSG. The quench effect is the result of a temperature difference between the header portion and the condensate. The quench effect increases stress within the HRSG. The increase stress ultimately results in a shorter operational life for the HRSG.